Some automatic door systems use optical sensors for controlling opening and closing operations of a door panel. An example of such automatic door systems is disclosed in Patent Literature 1. According to the technology of Patent Literature 1, detecting ranges for detecting the presence of a human or an object are set in outdoor and indoor sides of an door opening formed in a wall of a building. There are provided activating sensors for detecting the presence or absence of a human or an object in the detection ranges. An auxiliary sensor is disposed on jambs erected on both sides of the door opening. A light-emitter is disposed on one of the jambs, and a light-receiver is disposed on the other jamb. The light-emitter and the light-receiver are facing each other. The auxiliary sensor is employed for keeping the door panel open when the presence of a human or an object in the vicinity of the door opening is detected when the door is open. The detection is based on reception, by the light-receiver, of no light from the light-emitter because of the light from the light-emitter intercepted by the human or object.    Patent Literature 1: JP 2002-227525